The amino acid proline is one of the three amino acids that watson describes as having "special properties". what is special about proline?

Proline (Pro or P) is an amino acid that enters into the composition of proteins.

The structure of proline is a bit special since it is the only cyclic amino acid. Indeed, It has a side chain in which the R group on the alpha carbon is connected to the nitrogen. This arrangement makes it possible for proline to adopt backbone conformations that are not possible in other amino acids.

Its particular structure inserts a bend in the protein chains and breaks the α helices and the β sheets. It can, however, create particular helices, proline helices like those found in collagen.

Proline is a non-essential amino acid, synthesized by the body.